I've also done the startup code and an assembly language graphics library for developing on the 7800 with CC65. That means you can use "C" for the logic and AI and my library handles the sprites, sound, joystick etc. Three of my games already use it.
I've also developed a windows graphics conversion tool that allows you to draw sprites in BMP files and it will output graphics for 160A, 160B, 320A and 320B MARIA graphics modes.
I'm hoping to get the library and supporting tools released sometime soon.
If you need more help, sign up over at AtariAge (if you aren't already a member) and head over to the 7800 programing section :-
http://www.atariage.com/forums/forum/52-atari-7800-programming/